I've had some users on my Yammer network notice that when posting several pictures on a Yammer post that the picture that displays bigger than the rest is not the first or last picture added to the post. Is there some way to figure out which picture will be the 'spotlight' image? I can't seem to figure it out.

just did several tests and can confirm that it's not always the first/last. Also checked the filenames and couldn't find any logic there. Looks random, but it probably has something to do with how fast the pictures are processed, which makes it look random.

The only thing you can do now AFAIK is share this as feedback on uservoice. It makes sense to highlight the first picture. All the previews are using the same aspect ratio, making it easier to improve this.

Update: found a related thread here. There is a uservoice link there. Looks like it's based on the width of the pictures. Picture with max width is highlighted. It will be the first picture if they all have the same width. Haven't tested this to confirm, but I'm convinced as it makes sense. The biggest picture will probably look better when you're giving it more space.